Platinum, either black or carbon-supported, is a well-known catalyst for incorporation in gas-diffusion electrode and catalyst-coated membrane structures, for instance in fuel cell electrolysis and sensor applications. In the prior art, carbon-supported platinum is commonly produced from platinum colloid sols, for instance starting from a complex platinum sulfite acid, as described in U.S. Pat. No. 3,992,512 and U.S. Pat. No. 4,059,541. This method involves oxidation of the H3Pt(SO3)2OH species with hydrogen peroxide (H2O2), optionally in the presence of carbon, and requires a complicated pH regulation over long reaction times. As a result, an uneven and quite large particle size can result, especially for relatively high platinum loadings.
An alternative route consists of reduction of chloroplatinic acid (H2PtCl6) with S2O32− ions optionally in the presence of carbon: Pt colloidal particles are thus formed and absorbed on the carbon particles surfaces, but this adsorption is not straightforward, and high platinum loadings are especially difficult to obtain. Again, the formation of colloidal platinum is quite slow, so that large particle size and uneven size distribution are very likely to occur.
The situation is not substantially improved by the method disclosed in U.S. Pat. No. 4,392,927, wherein carbon-supported platinum is prepared by addition of chloroplatinic acid to an alkaline solution containing sodium carbonate. The formation of a non-stoichiometric hydroxide and the non-uniformity of the reaction throughout the solution result in an unsatisfactory average particle size and distribution thereof. These prior art methods have in common a quite low reaction ratio, with the formation of few precipitation nuclei centers that have enough time to grow to consistent particle size. Since electrocatalysis is known to be primarily a surface phenomenon, a large particle size results in a lower catalytic activity per unit of catalyst weight. In other words, the utilization factor of the expensive platinum metal is relatively low.